As an example of a related art position detection method using ultrasonic, electronic pen systems are recited in Patent Literature 1 through 3. These position detection systems comprise an electronic pen having a function of transmitting an ultrasonic signal of a fixed waveform and an infrared trigger signal in a fixed cycle and a reception unit which receives two signals transmitted, in which the reception unit specifies a position of the electronic pen from an arrival time point of the trigger signal and an arrival time point of ultrasonic.
Patent Literature 1: U.S. Pat. No. 4,884,552
Patent Literature 2: U.S. Pat. No. 6,118,205
Patent Literature 3: Japanese Patent Laying-Open No. H07-104063
Non-Patent Literature 1: Hiroshi Kashiwagi, “M-Sequence and Its Applications”, Mar. 25, 1996, Shokodo.
Measurement of an ultrasonic signal propagation time employs a system structure using a sound wave of a specific frequency which is barely heard, in which when enabling propagation distance measurement on the order of 2 to 3 meters, a frequency around 40 kHz is often used and for further extending a propagation distance, a frequency in proximity to the least audible 25 kHz is used. To the contrary, when a propagation distance is short such as a case, for example, where a range in which a position relationship of the A4 size can be found is sufficient, a frequency in proximity to 80 kHz is used. This is because attenuation in a case where ultrasonic propagates in space is so large that it is difficult to propagate a signal over a distance of several meters by 80 kHz and receive the signal at a sufficient S/N ratio. In addition, because the shorter the distance becomes, the higher position precision is required, it is more likely that a high-frequency signal whose wavelength resolution is small will be used. In recent years, as application of an ultrasonic propagation time measurement technique, more electronic pen systems have been introduced together with a conference/education infrastructure such as a white board and a projector.
While these systems are in general disposed in common offices and schools, in terms of power-saving, security and barrier-free, there is a case where a human body (moving body) sensor (generally called motion detector) is at the same time disposed for automatically turning on/off lighting. Although the sensors in general use infrared in many cases, its single use might cause erroneous operation, so that moving body sensing which makes use of the Doppler shift of ultrasonic has been used together. Since an intensity of ultrasonic used here which is transmitted from the side of a motion detector is far larger than that used in an electronic pen system, when an ultrasonic frequency used in the motion detector and an ultrasonic frequency used in the electronic pen system coincide with each other, the electronic pen system will determine that fixed ultrasonic is constantly transmitted, resulting in having a position of the electronic pen be constantly fixed to prevent normal drawing.
In addition, also in a case where ultrasonic frequencies are different from each other, since a system which in particular transmits and receives ultrasonic modulated based on a pseudo random signal requires a reception unit side to have a large frequency band to cause noise, normal drawing is prevented.
An object of the present invention is to selectively remove an ultrasonic signal transmitted from a motion detector or the like which will be noise to maintain normal operation of an electric pen system or the like when an ultrasonic frequency used in an apparatus using ultrasonic such as a motion detector and an ultrasonic frequency used in an ultrasonic propagation time measurement system such as an electronic pen system are close to each other.